Salinized lacustrine organic-rich shale influenced by marine incursions: Algal-microbial community, paleoenvironment and shale oil potential in the Paleogene Biyang Depression, East China

Salinized lacustrine organic-rich shale (SLORS) in the Paleogene has been considered as one of the most important shale oil reservoirs in China. Although deposition of the Paleogene SLORS in East China is generally concomitant with marine incursions, the influence of marine incursions on the algal-microbial community, paleoenvironment and shale oil potential of SLORS is still controversial. Here we examine this issue using a case of the Paleogene Biyang Depression (Nanxiang Basin) in East China, based on the integration of bulk geochemical, mineralogical, organic petrographic, biomarkers, elemental geochemical, and shale reservoirs analyses. Marine incursions were occurred during deposition of the SLORS in the Paleogene Biyang Depression as evidenced by the presence of marine organisms and marine-derived biomarkers (24-n-propylcholestanes). Lake transgressions, global sea-level rise, and extensional tectonic setting were responsible for the occurrence of marine incursions. The algal-microbial community change is associated with marine incursions, from dominance of green algae with a weak influence of marine incursions to dominance of red (brown) algae and coccolithophores with a strong influence of marine incursions, accompanied by enhanced bacterial activity. With the stronger influence of marine incursions, higher salinity, more reducing conditions, higher productivity, and lower detrital influx were present in the paleo-lake, resulting in the deposition of SLORS with higher shale oil potential, which was characterized by higher free shale oil contents, more abundant pores and fractures in nanoto micro-meter scale, and higher brittle mineral contents. In summary, our data suggest that marine incursions have a positive influence on the development of biodiversity in paleo-lakes, and the deposition of SLORS with high shale oil potential. Therefore, the influence of marine incursions should be fully considered during the palaeoecological reconstruction and the shale oil exploration of SLORS, especially during high sea-level stages. In addition, the results provide guidance for future profitable shale oil exploration in East China and other similar areas.

Automated summary

The study on the Paleogene Biyang Depression in East China shows that marine incursions have a positive influence on the development of biodiversity in paleo-lakes and the deposition of shale oil reservoirs with high shale oil potential.